A fluidized-bed catalytic cracking plant includes a reactor vessel, a vertical reactor riser having an upper outlet which is in fluid communication with a separator system arranged in the reactor vessel, and a regenerator vessel. During normal operation, regenerated catalyst particles and hydrocarbonaceous feed are supplied to the inlet end of the reactor riser in which catalytic cracking of the feed takes place to form a mixture of gaseous product and catalyst particles. The mixture leaves the reactor riser at a high temperature of between 500 and 540.degree. C. or higher. The mixture of gaseous product and catalyst particles is passed into the separator system where gaseous product is separated from catalyst particles. The gaseous product is removed from the upper end of the reactor vessel, and the catalyst particles are discharged to the lower part of the reactor vessel where they are stripped. Stripped catalyst particles are passed to the regenerator vessel where coke deposited on the particles during cracking is burnt-off at a high temperature to obtain combustion products and regenerated catalyst. The combustion products are removed from the upper end of the regenerator vessel and regenerated catalyst is re-used.
Such a fluidized-bed catalytic cracking plant is described in European patent application publication No. 488,549. The known separator system comprises a cyclone separator and an auxiliary cyclone separator to provide a two-stage separation. The cyclone separator comprises a vertical housing open at its lower end provided with a cover at its upper end having a central opening, an inlet duct for tangential entry of a mixture of gas and catalyst particles from the outlet of the riser reactor, a particles outlet duct communicating with the open lower end of the vertical housing, and a gas outlet duct having a vertical section which extends through the central opening in the cover and which has an outer diameter which is smaller than the diameter of the central opening. The central opening and the outer surface of the gas outlet duct define an annular inlet port.
The cyclone separator is arranged in the upper part of the reactor vessel, so that during normal operation stripping gas present in the reactor vessel can be drawn into the cyclone separator through the annular inlet port by the difference in pressure between the interior of the reactor vessel and the interior of the cyclone separator.
It is an object of the present invention to improve the efficiency of the known cyclone separator in particular at the level in the cyclone separator where the separation efficiency is low.